Change references to org.apache.brooklyn for Maven groupId

Should be merged once the _org.apache.brooklyn_ artifacts are available on the OSS Sonatype Nexus server:

- https://oss.sonatype.org/#nexus-search;quick~org.apache.brooklyn
Yes, we should switch over to Apache infrastructure for SNAPSHOT publishing now (the old group IDs were a blocker for that task) - I'll investigate setting that up. Once up and running, the new URL will be under https://repository.apache.org/content/repositories/snapshots/